You don't need withdraw your application, the people in AINP call center is not AINP officer , they know nothing !
some people in your situation before ! you can require to change in category with all supporting documents and explain letter
that people tell me : Hi, I did not actually change my NOC code, it was same as in EDS (2253). I requested for change in category to Engineering Occupations. I supported my request with Job offer letter, current pay stubs and job description and responsibilities. I used Electrical designer/drafter title.
he get his certificate on time! in your case, both way can work, withdraw or required change to Engineering Occupations.
good luck!